I have an existing double socket on a solid wall. I'm having 40mm insulation backed Thermaline board fitted on this wall next week. Question is, how do I handle the socket? There's enough slack on the cables to allow it to be fitted to the new board, but I'm not sure if I can just fit a 47mm steel wall box to the surface (slighty recessed) of the wall or do I need to use a plastic wallboard back box?